Fuel processors (FPs) are integrated with fuel cells (FCs) for onsite and onboard power generation. The process technologies employed in FPs have to be properly selected on the basis of fuel characteristics, impurity tolerances and operating specifications of FCs. Chemical, thermal and process parameters need to be given due consideration to ensure smooth FP–FC integration. The chemistry and catalytic reaction engineering of H2 generation in FPs and its utilization in FCs through electrocatalysis provide major challenges. The reactor configurations, extent of their miniaturization and their internal hydrodynamics and other design factors need to be considered for proper integration. This article highlights the current state of engineering knowledge in FP–FC integration and future prospects for achieving more efficient FP–FC systems for large scale onboard deployment.
Though the fuel sources like methanol or natural gas can be used directly in fuel cells which are compact in nature, such systems have several drawbacks viz., lower power density and operational efficiency, slow oxidation kinetics of many hydrocarbon fuels and the need for extensive purification to remove undesirable impurities. The fuel cells incorporating separate fuel processors (FP) provide widest possible primary fuel options to generate hydrogen of required purity for the fuel cell (FC). The FP– FC integrated systems have accordingly gained higher acceptability levels. Also, the hurdles in developing an extensive hydrogen infrastructure for FCs have led to the incorporation of onboard reforming facilities in the form of FPs consisting of reformers, sulphur traps, water gas shift units and appropriate CO cleanup systems for generating hydrogen from a hydrocarbon source which are integrated with fuel cells (FC) for onsite power generation. Their operational efficiency depends on chemistry, thermodynamics, FP and FC system configurations, impurities and other factors. The FC component accounts for 33% of space and cost. The FPs along with power conditioners, air supply units, thermal management systems, water treatment facilities and process controllers account for the rest of the plant cost. The configuration of FPs depends on the type of fuel being used and the type of FCs to which they have to be integrated. They need to be customized for specific situations. The FP–FC integration poses major technological and engineering challenges. The published literature on this subject is rather scanty. This paper reviews the current status of FP and FC technologies, the major factors impacting their integration and future challenges to be overcome to make them more efficient and user friendly.
2. Basic features of FP and FC systems
2.1. FC classification and characteristic features
A FC is an electrochemical device that converts the chemical energy of a fuel like hydrogen directly into electrical energy. The FC technologies continue to attract research attention worldwide for developing novel distributed power stations, fuel cell vehicles and standby power units . The FCs are classified according to the type of electrolytes employed in them as well as their operating temperature, fuel or oxidant used and cell design . The important types are AFC, PEFC, PEMFC, PAFC, MCFC and SOFC. They are at various stages of commercial deployment based upon their applications and operating regimes. The AFCs, used predominantly in space missions, are expensive since they require pure H2 and O2 as reactants. PAFCs are preferred for distributed and centralized power production systems. They have achieved the widest commercial application. The PEFCs and PEMFCs which are the smallest and lightest of the designs are being employed in transport systems particularly for electrical vehicles. The recent developments on high temperature (100–200 1C) PEMFCs  have made it possible to achieve greater power efficiency and wider range of fuel usage in them. Their CO tolerance lies between 3000 and 5000 ppm with membrane durability of 20,000 h. They do not require humidified environment and are costed relatively less than, their low temperature counterparts. With increased fuel flexibility and greater level of impurity tolerance, MCFCs and SOFCs which operate at hottest temperatures, are favoured for co generation or waste heat based power plants by taking advantage of the excess heat generated. Table 1 highlights FC characteristic features viz., operating temperatures, energy conversion efficiency, power application range, preferred application areas and desired impurity tolerances in hydrogen feed. Their materials of construction in determine their versatility or limitations in application . Reaction engineering aspects of FCs will enable understanding of their nonlinear dynamic behaviour and mass and heat transfer problems . The sensitivity of metal catalysts employed in them and their impurity tolerances have profound influence while evolving FP–FC integration strategies. They require ancillary facilities like manifolds, blowers, heat exchanges, humidifiers, condensers, valves and filters. Each application requires careful selection of hardware and software for achieving optimal performance.
2.2. FP chemistry and process technologies
FPs can utilize a variety of gaseous, liquid and solid fuels with H2 as the common reductant. The rapidly increasing fuel costs and stringent GHG emission regulations have intensified the search for more eco-compatible fuel options . Fig. 1 indicates the currently available lower carbon primary and secondary fuel options. The latter are employed as energy sources for FPs. The FP–FC systems employed for combined heat, hydrogen and power operates on hydrogen rich gases from anaerobic digestion of municipal waste water. Among the oxygenates, methanol (SE: 5.5 and ED: 4.4) is the most popular fuel for reforming since it requires mild process conditions and has the potential to attain highest conversion efficiency. Ethanol is gaining popularity for its ecofriendliness as well sustainability. Among the gases, natural gas (SE: 13.9 and ED: 2.3), propane and LPG are the most attractive fuels for FPs because of their large scale availability and high conversion efficiencies. Lee et al.  presented a compact and highly efficient natural gas FP consisting of a reformer and a water gas shift reactor. Eventhough, gasoline and diesel (SE: 12.6 and ED: 10.6) are attractive fuels due to their high specific energy density and easy availability, several S&T barriers have to be overcome for their deployment in FCs. Biomass (SE: 4.2 and ED: 3.0) and other solid fuels provide potential future options due to their carbon cycling neutrality and renewability. Synthetic liquid fuels from coal, biomass and natural gas are produced either by direct or indirect conversion process. Woody biomass can be pyrolysed to produce bio-oil which is deoxygenated to produce fuel suitable for FC application.
The Fuel processors have to be carefully designed to generate a relatively uniform hydrogen rich gas stream for a given FC [2,7,9,10] from the wide range of secondary fuels highlighted in Fig. 1. They contain widely varying classes of unwanted compounds that must be removed with different type of catalysts and associated post treatment processes. Some of the fuels are difficult to reform. Accordingly, a variety of reforming options have been reported (Fig. 2). Most widely employed process is steam reforming to produce synthesis gas with a high H2/CO ratio. The auto thermal reforming option employs endothermic reforming along with exothermic oxidation reaction to provide a thermal balance. The steam reforming option requires more startup time than its autothermal counterpart. However, it provides H2 concentrations above 75% as compared to 50% achieved by the autothermal option. The presence of catalyst poisons in the fuels add to the reforming process complexities and curtailment of their overall efficiency . The sorbent enhanced reforming is an emerging technology for the production of high purity hydrogen from hydrocarbons with insitu CO2 capture . The ion transport membrane reforming is an important platform technology proposed for natural gas as fuel. It combines air separation and partial oxidation or autothermal reforming in a membrane reactor . Plasma reformers avoid the use of noble metal catalysts for gasoline or natural gas reforming. The core components are arc electrodes and a microwave nozzle for producing a stable are torch . Pressure swing reforming process has also been reported .
ThIn actual practice the overall reaction scheme for reforming of hydrocarbon fuels to hydrogen comprises a combination of 4 other reactions coupled with reforming viz., catalytic partial oxidation, water gas shift reaction, preferential oxidation of CO and methanation. A compact FP system needs a combined reforming and insitu partial oxidation on the same catalyst bed to avoid any external heat source for the endothermic reforming process. Another pre-reforming operation that may be required for C2–C6 hydrocarbons is hydrocracking to provide better reforming characteristics. Several technological options are thus available for small scale reformers for portable FP–FC systems. Ogden reviewed  their potential for integration with FCs.e Fuel processors have to be carefully designed to generate a relatively uniform hydrogen rich gas stream for a given FC [2,7,9,10] from the wide range of secondary fuels highlighted in Fig. 1. They contain widely varying classes of unwanted compounds that must be removed with different type of catalysts and associated post treatment processes. Some of the fuels are difficult to reform. Accordingly, a variety of reforming options have been reported (Fig. 2). Most widely employed process is steam reforming to produce synthesis gas with a high H2/CO ratio. The auto thermal reforming option employs endothermic reforming along with exothermic oxidation reaction to provide a thermal balance. The steam reforming option requires more startup time than its autothermal counterpart. However, it provides H2 concentrations above 75% as compared to 50% achieved by the autothermal option. The presence of catalyst poisons in the fuels add to the reforming process complexities and curtailment of their overall efficiency . The sorbent enhanced reforming is an emerging technology for the production of high purity hydrogen from hydrocarbons with insitu CO2 capture . The ion transport membrane reforming is an important platform technology proposed for natural gas as fuel. It combines air separation and partial oxidation or autothermal reforming in a membrane reactor . Plasma reformers avoid the use of noble metal catalysts for gasoline or natural gas reforming. The core components are arc electrodes and a microwave nozzle for producing a stable are torch . Pressure swing reforming process has also been reported .
The removal of CO from reformate to meet the allowable limits of FCs is one of the major challenges of FP–FC integration. Water gas shift reaction (WGSR) converts CO to form CO2 and H2 and is employed for CO removal from the reformates to the extent of 0.5– 1%. It is a temperature sensitive reaction and is often employed as high (350þ 1C) and low (190–210 1C) temperature options. Iron oxide promoted by chromium oxide and zinc/aluminium oxide catalysts are employed in HTS and LTS options respectively. Noble metal coated inorganic membranes are employed in WGSR for further improving hydrogen purity. A detailed review on this option is published recently  by the authors. The preferential oxidation technology enables CO oxidation on a heterogeneous catalyst to reduce it to 10 ppm level employing excess of oxygen (factor of 2). The instrumentation and process control strategies have to be worked out . Catalytic CO methanation is another option for the purification of hydrogen rich gas mixture . It is the reverse of steam methane reforming with no requirement of air. The CO concentration can be reduced from 0.5% to less than 20 ppm. The high temperature gasification/pyrolysis of solid fuels followed by the catalytic reforming of gas/liquid product is an emerging technological option for FPs . Due to the difficulty in volatilizing the biomass based fuels, technologies based on aqueous phase reforming and supercritical water oxidation are receiving attention .
From engineering considerations, a FP systems consists of a three tubular reactors with each of them modelled as an isothermal plug flow reactor with minimal axial heat and mass transfer. The automotive onboard application places severe constraints on their volume. Kolavennu et al.  reported their design strategies.
2.3. FC and FP market opportunities
The fuel cell industry achieved its first commercial success in 2007 [20,21]. A significant growth in annual FC shipments worldwide has been reported since then. The global shipments were 80,000 units (63% stationery, 33% portable and 4% transport) in 2012 with more than threefold increase recorded in a single year. In megawatt terms, the installed capacity was 180 MW. This is due to a surge in the growth of combined heat and power units and portable consumer electronics chargers. The early market applications of material handling, backup power, residential CHPs and portable and auxiliary power packs have been achieved by 2011 . However, fuel cell applications in transport sector continue to fluctuate on cost and compactness constraints inspite of their process technology successes. The PEMFC based UPS systems are now commercially available from 65 W to 400 kW range. 1 MW distributed stationary power systems based on PEMFCs have been commercialized in North America. 1 kW combined heat and power residential fuel cell systems based on LPG are undergoing field trials in Denmark. It is reported that 4–20 kW full find use in material handling and ground support equipments. Fuel cell power modules (75 W–150 kW) for hybrid buses are in use in North and South America and Western Europe. With reference to FPs, production of smaller reformer systems is an attractive midterm option for integrated FP–FC systems for the automotive sectors in regions where low cost natural gas is readily available . Steam, and autothermal reforming, partial oxidation and catalytic cracking of methane and ammonia for FPs are now receiving attention for large scale deployment. Multifuel FPs (suitable for 5 kW FCs) which can handle natural gas and a range of gaseous fuels have recently been field tested in Japan in residential sector.